1. Field of the Invention
The invention relates to discs which protect fluid systems from excessive pressure by rupturing at a pre-determined pressure so as to release fluid from the system thereby lowering the pressure.
2. Description of the Prior Art
Overpressure relief devices have been used for many years to provide protection of equipment and people against excessive fluid pressure in a system or vessel. A common type of overpressure relief device uses a disc that is designed to rupture at a predetermined pressure. It is, in effect, a planned point of weakness in the pressure system. When the disc ruptures, the fluid which has the excessive pressure passes through the rupture opening and is either collected or discharged into the surroundings. This avoids a catastrophic failure of the system or vessel at another point.
It has been found that a cupped disc, with the convex side facing in the direction of the potential excess pressure, works well for this purpose. It will withstand for long periods pressures that are near to the predetermined rupture pressure, and will usually rupture immediately when the predetermined pressure is exceeded. Since the process of rupturing involves a reversal of the direction of convexity of the disc, this type of disc may be described as a reverse-buckling rupture disc.
The usual manufacturing procedure is to produce a batch of identical discs made of identical material, and then test some of them to discover the pressure at which typical discs in the batch will rupture. The characteristics of the tested discs are ascribed to the remaining discs in the batch, which are then put on the market.
Discs that are designed to rupture from overpressure on the concave side are also known in the art, and may be described as pre-bulged rupture discs. The present invention does not concern these, but is instead directed to reverse-buckling rupture discs.
A problem with previous rupture discs is that the rupture sometimes fails to produce a large hole and therefore fails to provide a definite and complete release of the pressure. A further problem is that discs sometimes fragment upon rupturing, and thereby release pieces of metal that may be dangerous to people or to the part of the system provided to collect the discharged fluid.
The completeness and cleanness of the rupture of the reverse-buckling rupture discs has previously been enhanced in two ways. One way is the provision of knife blades on the concave side of the disc, so that when the disc reverse-buckles, the knife blades slice the disc and thereby ensure that the opening is large and clean. Another way is the scoring of the disc during the manufacturing process, thereby introducing lines of weakness that are lines of potential cleavage when reverse-buckling occurs.
Two disadvantages of the method of providing knives are that the knives become dull after a number of ruptures or as a result of corrosion, and they may be dislodged and become inoperative. Both these problems cause the system to be unreliable. A third disadvantage of using knives is that an additional structure to support the knives adds to the size, weight and complexity of the device. A fourth disadvantage is that the method of using knives is costly in the manufacturing stage and costly for the periodic maintenance of sharpening the knife edges.
The principal disadvantage of a disc which has been scored is that the completeness of rupturing is not highly reliable. Scoring may improve the reliability of complete opening, but there are occasions when a scored disc does not open completely and therefore impedes the release of the pressure. Another disadvantage of scoring the discs is that the manufacturing process is troublesome and is costly because of the frequent need to maintain the instrument used to make the scores. The scores may also have an effect on the rupture pressure which is difficult to predict, although in a reverse-buckling rupture disc the effect is probably small because the main factors which determine the rupture pressure are the nature of the disc material, the thickness of the disc, and the pressure used to make the bulge in the disc.
The present invention discloses a type of reverse-buckling rupture disc that is manufactured in a way that ensures complete opening without fragmentation when the disc reverse-buckles. As soon as a certain pressure is reached, the disc reverse-buckles and after reversing it ruptures almost instantaneously.